The present invention discloses an effective technique to provide protection to high frequency circuits such as, but not limited to, low-noise amplifiers (LNA""s) and millimeter wave integrated circuits (MMIC""s) from electrostatic disturbance and potentially damaging high-power signals utilizing a microelectomechanical (MEM) device.
In the construction of high-frequency integrated circuits, including MMIC""s, power limiters are used at the input of circuits including low noise amplifiers to prevent device burnout from undesirably high levels of incident RF power. PIN diodes are typically used as power limiters, but these diodes are lossy, particularly at millimeter-wave frequencies. Further, diodes are difficult to use as they require impedance matching to the circuitry to which they are connected and tend to break down at very high power levels. Any loss due to a power limiter adds directly to the noise figure of the circuit, resulting in reduced sensitivity to desired signals and greater power requirements for the system resulting from additional complexities of design. Additionally, it is often difficult to monolithically integrate PIN diodes with transistors in a single process while the present invention may be integrated onto the same substrate as active devices such as transistors in a high-frequency integrated circuit process.
The present invention overcomes many of the difficulties found in the use of diodes as power limiters by providing a flexible mechanical bridge over a transmission line on the substrate which utilizes the electromagnetic field increase generated by temporary increases in power to short the harmful signal away from the remainder of the circuit.
Semiconductor devices are sensitive to excessive input voltages, such as those generated by ESD. High-speed devices are particularly sensitive. Circuits and systems that encounter ESD typically suffer from either immediate or latent component failure. In low frequency applications, the most common technique for protecting the input/output/power pins from damage is to include ESD diodes to shunt the undesired input signal away from the active devices and a series resistor to allow for sufficient time for the diodes to turn on. However, ESD diodes tend to have a large capacitance which prohibits their use in RF/microwave applications, and the series resistor is not acceptable in this type of system due to the incurred loss. The result of these shortcomings in diodes and resistors leave the typical high-speed devices, which operate at high frequencies, unprotected.
In contrast, the present invention sets forth a method to utilize a mechanical cantilever type switch to serve as protection from ESD.
In accordance with the present invention, a MEM implementation of a power limiter is presented, utilizing the electromagnetic field increase caused by a substantial increase in power through a transmission line on a substrate to cause the mechanical flex of a strip of conductive material traversing the transmission line. Upon flexion, the conductive material contacts the microstrip and provides a path by which the signal is shorted to ground. As a result, devices further down the circuit are protected from damage. The MEM power limiter is low loss and can easily be integrated with low noise active devices such as HEMT""s or HBT""s in MMIC""s. The MEM limiter is intentionally designed to actuate at high RF inputs to protect the active devices from damagingly high signals. Although the speed of the MEM power limiters will typically be less than that of PIN diode limiters, by proper design of the limiter it is possible to protect the active devices from burnout.
Also presented in accordance with the present invention, is a MEM implementation of a cantilever type switch activated by an on-board signal from an active circuit such as a MMIC which may be used to as a safety mechanism to protect high speed devices from excessive input voltages or as a switch for other purposes such as an on/off switch. The advantage of the MEM cantilever type switch is that it is causes very low losses, thereby facilitating the protection of microwave devices in a manner that does not appreciably degrade their normal performance.